One objective is to study seasonal and thermal induced changes in ventilatory control of ponies. Cold acclimatized ponies will be studied in cold (3 degrees C) and hot (23 degrees C) environments, and 6 months later the same ponies acclimatized to hot conditions will be studied in cold (3 degrees C) and hot (23 degrees C and higher) environments. These studies involve measurements during eupnea, during CO2 exposure, and during muscular exercise. Our second goal is to determine the ventilatory effects of altered PICO2 in pulmonary airways of ponies. Six normal ponies will be studied during exposure to various PICO2. We will then create a low trachestomy and a double lumen T tube will be inserted into the trachea so that the pony can breathe either through the side port of the tube or through the airway in a normal fashion. After 4-5 days of recovery from surgery, studies will be completed at various levels of PICO2 with equal numbers of studies while the tracheal port is open and closed (i.e. airway intact or bypass upper airways). Environmental temperature will be set so that studies can be completed on tachypneic and nontachypneic ponies. Finally, we will attempt through bronchosphirometry to bypass even more of the airways, and to then determine the effect of this procedure on ventilatory response to altered PICO2. Our third goal is to analyze transthoracic impedance tracings for VE, Vt, and ventilatory timing. These tracings have been obtained during our recent studies on ponies over numerous conditions. Each of these conditions causes marked changes in the variables and the analysis we propose will provide valuable insight into the subcomponent of the respiratory center affected by the various conditions.